In U.S. Pat. No. 3,696,391, issued in the name of Georges Peronneau, there has been disclosed a system of this type including a marker which can be pointed at a selected part of a display of an oscilloscope screen which an operator wishes to alter or replace. The marker may be a light pen designed to pick up a luminous spot on the screen and to signal its co-ordinates to the computer which determines therefrom the identity of the component to be modified, allowing the operator to change the parameters or to choose a different component with the aid of a keyboard.
The aforedescribed technique is particularly applicable to a system in which the individual image components are traced by random scanning, under the control of analog voltages emitted by the aforementioned function generator, since in that case the code words relating to the several image components are sequentially read out from memory into a register of a control unit in which they are stored for the duration of the tracing operation so as to be instantly available whenever the marker touches a point on the oscilloscope screen illuminated exclusively the the corresponding trace. However, random-scanning systems have the drawback that the time required for tracing a particular character varies with the information content thereof, i.e. with the number and complexity of the constituent image elements. Thus, the renewal rate of such a character depends on this information content and varies therefore from one character to another, with a consequent fluctuation of luminous intensity. Moreover, it is difficult if not impossible in such a system to superimpose images from different sources on one and the same screen by electronic means.
With an orthogonal or raster-type scan as used in television reception, on the other hand, figures of any configuration can be reproduced at a uniform recurrence rate of, say, 50 frames per second. During each frame period, a pulse pattern encompassing all the image components of the display is present in a refreshing memory such as a storage tube. Thus, a code word identifying an individual component of that display is not necessarily present in a register of the processor at the instant when a light pen is pointed at a corresponding part of the oscilloscope screen.